This invention relates to laser systems nd more particularly to a multiple crystal, single pumping cavity laser system, with means for thermally and mechanically isolating the system pumping cavity from other components of the system.
Typical pulsed solid state lasers, which use crystals such as ruby, ND YAG, Alexandrite, or glass, consist of an oscillator system followed by one or more amplifier systems. Each amplifier system in such a scheme has its own respective optical pumping cavity containing a single crystal, flash lamps, reflectors, and other components powered by its own capacitor bank, power supply and pulse forming network. The advantage of such a modular system is that the total system can be tuned to provide optimum amplification while maintaining oscillator pumping at a desirable level.
In applications where the TEM,, mode is desirable, the oscillator crystal is optically pumped to fire slightly above the lasing threshold, in order to limit the creation of unneeded modes in the oscillator cavity. Inserted into the oscillator cavity are a number of mode selection devices, such as etalons, intracavity apertures, Q-switches, and other devices, all of which diminish the power output from the oscillator to between 10-30 millijoules in the typical case. However, many applications, such as holography, require much higher energy levels of between 1-10 joules. An increase in energy is achieved by passing the oscillator beam through one or more amplifiers, which, for various reasons usually contain crystals of increasing diameter. The diameter is determined by the energy level at that point in the amplifier chain, which must be kept below the damage threshold of the particular crystal. It is usually necessary to slowly diverge the beam leaving the oscillator by inserting a negative or positive lens in such a manner that the beam nearly fills the exit diameter of each of the rods. Since the crystals and the amplifier chain have varying diameters, the crystals must be optically pumped at different levels to attain optimum amplification. This can be achieved by changing the energy of the respective power supply that is pumping the capacitor bank for that particular amplifier.
In such a system, it is a common practice to insert a time delay device between the trigger pulse source and the oscillator. The first amplifier receives a trigger pulse directly and fires first, followed by firing of the oscillator, so that the amplifier crystal, with its longer flash lamp event, has time to be fully pumped before the oscillator pulse passes through the amplifier crystal. If there are significantly different flash lamp configurations in various amplifier stages, then further time delay devices are inserted and delay times are adjusted for maximum amplification at each stage.
It is important that the amplifier stages are arranged in such a manner that there is no feedback, parasitic oscillation, prelasing, or super-radiance caused by reflections from the end faces of the crystals. Such problems can be obviated by employing one or more methods, including offsetting the crystal from the main optical axis, anti-reflection coating the crystals, cutting the crystal ends at an angle, inserting Q-switching devices such as dye cells or electrooptical switches, and/or using optical rotators. However, implementing these solutions involves significant cost and/or additional mechanisms. It is also important to safeguard all of the optical surfaces in the amplifier chain from dust, which, due to the high energies involved, will incinerate and cause optical damage.
Thus, in order to obtain a relatively high energy laser output signal of between 1-10 joules, it has been necessary to provide an oscillator system having several amplifier systems, each having its own flash lamp or lamps, reflectors, capacitor bank, power supply and pulse forming network. The resultant system, with its duplication of components, is not only highly costly, but takes up a considerable work area due to its relatively large size, and often requires that the subject matter be brought to the laser, which may be inconvenient or impractical. Moreover, the overall efficiency of the system is low, due to the significant losses in the respective circuits, flash lamps, heating of the pump cavity walls and crystals, and florescence decay.
Certain applications of solid state pulse lasers, such as commercial holographic lasers, depend for their successful use on achieving a repeatable and precisely tuned TEM.sub.00 single frequency output. According to the prior art, pulsed laser oscillators have been constructed by mounting all of the oscillator cavity components (including a front mirror or etalon, intracavity aperture, polarizer, Q-switch, intracavity etalon, rear reflector, and pumping chamber) on a common rigid bed, on a laser rail, or in more sophisticated versions, on a three-point invar rail system. In practice, a TEM.sub.00 monomode output is both difficult to achieve and sustain with prior systems. It has been discovered that the pumping cavity generates a significant amount of thermal and mechanical energy, which is transmitted to other components of the oscillator system. A repeatable TEM.sub.00 single frequency pulsed laser output is dependent primarily upon the maintenance of parallelism and a precise distance of the resonator mirrors, as well as the surfaces of any etalons within a fraction of a wavelength of light and a second of arc. Accordingly, thermal and mechanical shocks generated in the pumping chamber due to the discharge of high energy flashlamps cause the system to lose parallelism, loss of alignment and fine tuning.